The present invention relates to a process for the preparation and purification of nitroaromatics.
Nitroaromatics constitute valuable intermediates for dye synthesis, primary products for plastics (e.g., toluylenediisocyanate and diphenylmethanediisocyanate), plant protection products, etc. Such nitroaromatic compounds are generally reduced to the corresponding amines, which are then further processed.
Nitroaromatics are prepared industrially by reacting aromatic hydrocarbons with nitric acid in the liquid phase in the presence of a Lewis acid catalyst. Commodity chemicals, especially, nitrobenzenes and nitrotoluenes having one or more nitro groups, are obtained in this manner. Mixtures of sulfuric acid and nitric acid of varying concentrations are used for the reaction. The literature also describes processes which work with only nitric acid (U.S. Pat. No. 4.918,250; U.S. Pat. No. 5,001,272 and WO 89/12620) or N.sub.2 O.sub.4/ O.sub.2 mixtures (EP 169 441 and EP 173 131) in the liquid phase.
Common features of the known processes for producing nitroaromatics are: 1) the nitroaromatics are separated from the aqueous acid and the water produced by the reaction, and 2) neutralization of the acids dissolved in the nitroaromatic is performed with basic substances (e.g. with metal oxides, metal hydroxides, metal carbonates, metal hydrogen carbonates, ammonia, amines, etc.). These bases are used in the form of aqueous solutions in order to wash the nitroaromatic free of acid. The nitroaromatic is further scrubbed with fully deionized water in order to remove dissolved salts from the nitroaromatic. A considerable effluent volume (approx. 50 to 1000 kg effluent per tonne of nitroaromatic) results. This effluent is heavily contaminated with inorganic salts such as nitrates, sulfates and nitrated aromatic compounds (nitroaromatics, nitrophenol derivatives, nitrobenzoic acids, etc.).
Nitrated aromatic compounds are known to be difficult to degrade in biological treatment plants. Inorganic nitrates are partly responsible for the increased nitrogen input to the natural environment. Effluents from the production of nitroaromatics must therefore be extensively treated in order to remove these pollutants before leading to a waste water treatment plant.
Another possible route for separation of the aqueous acid from the nitration mixture is the crystallization of the nitroaromatics out of the solution. For example, U.S. Pat. No. 5,057,632 describes the possibility of separating dinitrotoluene from the reaction solution by single-stage crystallization. However, this separation technique changes the isomer ratio in the dinitrotoluene. Further purification of the crystallized dinitrotoluene to separate from normally present inorganic and organic acids is not described.
DE 2 926 947 discloses that nitrosation agents can be removed from the nitroaromatics after nitration of the nitroaromatics by crystallization. However, it is then necessary to separate and wash the compound which crystallizes out with a pH-neutral material, in order to remove other organic and inorganic acids from the product.